1. Field of the Invention
The present invention relates to improved composite materials and the melt infiltration methods for producing the same. Specifically, the present invention relates to silicon-based composites wherein potentially large components of potentially complex shape can be made accurate dimensionally, reliably, and utilizing relatively low processing temperatures. In a preferred embodiment, the silicon-based composite contains silicon carbide, at least a portion of which is produced by reactive infiltration.
2. Discussion of Related Art
Silicon carbide (SiC) composites have been produced by reactive infiltration techniques for decades. In general, such a reactive infiltration process entails contacting molten silicon (Si) with a porous mass containing silicon carbide plus carbon in a vacuum or an inert atmosphere environment. A wetting condition is created, with the result that the molten silicon is pulled by capillary action into the mass, where it reacts with the carbon to form additional silicon carbide. This in-situ silicon carbide typically is interconnected. A dense body usually is desired, so the process typically occurs in the presence of excess silicon. The resulting composite body thus contains primarily silicon carbide, but also some unreacted silicon (which also is interconnected), and may be referred to in shorthand notation as Si/SiC. The process used to produce such composite bodies is interchangeably referred to as “reaction forming”, “reaction bonding”, “reactive infiltration” or “self bonding”.
Reaction bonded silicon carbide (sometimes referred to in shorthand notation as “RBSC”) ceramics combine the advantageous properties of high performance traditional ceramics, with the cost effectiveness of net shape processing. Reaction bonded silicon carbide ceramic offers extremely high levels of mechanical and thermal stability. It possesses high hardness, low density (similar to Al alloys) and very high stiffness (˜70% greater than steel). These properties lead to components that show little deflection under load, allow small distances to be precisely controlled with fast machine motion, and do not possess unwanted low frequency resonant vibrations. In addition, due to the high stiffness and hardness of the material, components can be ground and lapped to meet stringent flatness requirements. Moreover, as a result of very low coefficient of thermal expansion (CTE) and high thermal conductivity, RBSC components show little distortion or displacement with temperature changes, and are resistant to distortion if localized heating occurs. Furthermore, both Si and SiC possess refractory properties, which yields a composite with good performance in many high temperature and thermal shock applications. Finally, dense, high purity SiC coatings can be applied when extremely high purity and/or superior resistance to corrosion are required.
In one of the earlier demonstrations of this technology, Popper (U.S. Pat. No. 3,275,722) produced a self-bonded silicon carbide body by infiltrating silicon into a porous mass of silicon carbide particulates and powdered graphite in vacuo at a temperature in the range of 1800 to 2300° C.
Taylor (U.S. Pat. No. 3,205,043) also produced dense silicon carbide bodies by reactively infiltrating silicon into a porous body containing silicon carbide and free carbon. Unlike Popper, Taylor first made a preform consisting essentially of granular silicon carbide, and then he introduced a controlled amount of carbon into the shaped mass. In one embodiment of his invention, Taylor added the carbon in the form of a carbonizable resin, and then heated the mass containing the silicon carbide and infiltrated resin to decompose (carbonize) the resin. The shaped mass was then heated to a temperature of at least 2000° C. in the presence of silicon to cause the silicon to enter the pores of the shaped mass and react with the introduced carbon to form silicon carbide.
In spite of the many outstanding properties, including high specific stiffness, low coefficient of thermal expansion, and high thermal conductivity enumerated above, reaction bonded SiC ceramics generally have low fracture toughness, and therefore may not be optimal in applications where impact loading will occur.
In response, materials investigators have experimented with various techniques for enhancing the toughness or impact resistance of such inherently brittle ceramic-rich materials. Perhaps the most popular approach has been to incorporate fibrous reinforcements and attempt to achieve crack deflection or fiber debonding and pull-out mechanisms during the crack propagation process.
Hillig and his colleagues at the General Electric Company, motivated in part by a desire to produce silicon carbide refractory structures having higher impact strength than those of the prior art, produced fibrous versions of Si/SiC composites, specifically by reactively infiltrating carbon fiber preforms. See, for example, U.S. Pat. No. 4,148,894.
More recently, German Patent Publication No. DE 197 11 831 to Gadow et al. disclosed a reaction-bonded silicon carbide composite body featuring high heat resistant fibers, in particular those based on silicon/carbon/boron/nitrogen, for example, carbon or silicon carbide. The composite body was formed by the melt infiltration of a silicon alloy into a porous preform containing the fibers. The alloying element for the silicon-based infiltrant may consist of iron, chromium, titanium, molybdenum, nickel and/or aluminum, with iron and chromium being preferred, and with 5-50% iron and 1-10% chromium being particularly preferred. The alloying addressed the problem of the jump-like internal strain caused by the volume increase of silicon upon freezing. Previously, in large or thick-walled articles, this cooling strain was sufficiently large in many cases as to manifest itself as microfractures throughout the composite body. Thus, the stability of the material was reduced, and a critical growth of the fractures was to be expected under application of alternating thermal and mechanical stress. Accordingly, by alloying the silicon phase, the jump-like strain was reduced or even avoided, thereby solving the problems associated with the silicon cooling strain. The exchange of some brittle silicon for a different metal also led to a clear increase in toughness and ductility of the composite body.
At a minimum, the matrix of Gadow et al. contains iron. In a further refinement, it is preferred to add to the iron-containing silicon matrix, further additives of chromium, titanium, aluminum, nickel or molybdenum in a suitable ratio for the formation of a passivation layer, so that it results in improved oxidation resistance and corrosion resistance. With specific regard to the aluminum addition, it is known from ferrous metallurgy that aluminum is never present in iron-based alloys in amounts more than about one or two percent. This is because aluminum is chemically reactive with iron, and additions of aluminum to iron will tend to form iron aluminides rather than result in elemental aluminum dissolved in iron.
In spite of the toughening afforded by the alloying, Gadow et al. still rely on fibrous reinforcement. In fact, they attribute part of the strength of the composite to its fibrous reinforcement, and the fact that they treated the fibers gently during the granulation process so as to not damage them and thus impair their strength. Fibers, particularly fibers based on silicon carbide, can be expensive. Further, short fibers such as chopped fibers or whiskers, can pose a health hazard, and efforts must be taken to insure that such fibers do not become airborne or breathed. Fibers are often added to a ceramic composition to enhance toughness through debonding and pull-out relative to the matrix. If another way could be found to toughen the silicon carbide composite bodies of interest, then one could dispense with the fibers.
Further, at least some of the infiltrant alloy compositions disclosed by Gadow, such as Fe35-Si65 alloy, have a melting point below that of pure silicon, and it would seem possible and even advantageous to take advantage of this phenomenon. Gadow acknowledges the lower melting point, but fails to take advantage of it, and instead recommends infiltrating at temperatures well above the silicon melting point, such as at 1550° C. and 1700° C., in his Examples 1 and 2, respectively.
Chiang et al. (U.S. Pat. No. 5,509,555) disclosed the production of composite bodies by a pressureless reactive infiltration. The preform to be infiltrated by the alloy can consist of carbon or can consist essentially of carbon combined with at least one other material such as a metal like Mo, W, or Nb; a carbide like SiC, TiC, or ZrC; a nitride like Si3N4, TiN or AlN; an oxide like ZrO2 or Al2O3; or an intermetallic compound like MoSi2 or WSi2, or mixtures thereof. In any event, the preform bulk density is rather low, about 0.20-0.96 g/cc. The liquid infiltrant included silicon and a metal such as aluminum, copper, zinc, nickel, cobalt, iron, manganese, chromium, titanium, silver, gold, platinum and mixtures thereof.
In a preferred embodiment of the Chiang et al. invention, the preform could be a porous carbon preform, the liquid infiltrant alloy could be a silicon-aluminum alloy containing in the range of from about 90 at % to about 40 at % silicon and in the range of from about 10 at % to about 60 at % aluminum and the carbon preform could be contacted with the silicon-aluminum alloy at a temperature in the range of from about 900° C. to about 1800° C. for a time sufficient so that at least some of the porous carbon reacted to form silicon carbide. Upon cooling, the dense composite formed thereby can be characterized by a phase assemblage comprising silicon carbide and at least one phase such as silicon-aluminum alloy, a mixture of silicon and aluminum, substantially pure aluminum or mixtures thereof.
One problem with infiltrating multi-constituent liquids into preforms containing large fractions of carbon is that the infiltrant chemistry can change dramatically over the course of infiltration, as well as from one location to another within the preform. Table 3 of Chiang et al. demonstrates this point. There, the infiltrant started out as being about 54 at % Si, 46 at % Cu, but after infiltration into a carbon preform, it was substantially 100% Cu. Such drastic compositional changes can make processing difficult; this same Table revealed that when the infiltrant alloy started out at about 30 at % Si, 70 at % Cu, pressure was required to achieve infiltration. Pressure infiltrations require much more complex and expensive equipment than do pressureless infiltration techniques, and usually are more limited in the size and shape of the parts that can be produced thereby. Thus, while the present invention is not limited to pressureless systems, unless otherwise noted, the infiltrations of the present invention refer to those not requiring the application of pressure.
Chiang et al. stated that their method allows production of composites very near net-shape without a need for additional machining steps. They described a number of non-machining techniques for removing the residual, unreacted liquid infiltrant alloy remaining on the reacted preform surface. Specifically, Chiang et al. stated that following infiltration, the composite body could be heated to a temperature sufficient to vaporize or volatilize the excess liquid alloy on the surface. Alternatively, the reacted preform could be immersed in an etchant in which the excess unreacted liquid infiltrant is dissolved while the reacted preform is left intact. Still further, the reacted preform could be contacted with a powder that is chemically reactive with the unreacted liquid infiltrant alloy such as carbon, or a metal like Ti, Zr, Mo or W.
In U.S. Pat. No. 5,205,970, Milivoj Brun et al. also was concerned with removing excess infiltrant following production of silicon carbide bodies by an infiltration process. Specifically, Brun et al. contacted the reaction formed body with an infiltrant “wicking means” such as carbon felt. More generally, the wicking means could comprise porous bodies of infiltrant wettable materials that are solid at the temperature at which the infiltrant is molten. Preferably, the wicking means has capillaries that are at least as large or larger than the capillaries remaining in the reaction formed body. Thus, infiltrant in the reaction-formed body that was filling porosity remained in the reaction formed body instead of being drawn into the wicking means and leaving porosity in the reaction formed body. The infiltrant could be silicon or a silicon alloy containing a metal having a finite solubility in silicon, the metal being present up to its saturation point in silicon.
The “wicking means” solution of Brun et al. to the problem of removing excess adhered silicon, while perhaps effective, nevertheless requires the additional processing steps of contacting the formed composite body with the wicking means and re-heating to above the liquidus temperature. What is needed is a means for eliminating or at least minimizing the degree of residual infiltrant adhered to the formed composite body.
Thus, it is an object of the present invention to produce a silicon-containing composite body of improved toughness, preferably without reliance on fibrous reinforcement as a toughening mechanism.
It is an object of the present invention to produce a composite body by an infiltration process whereby the residual infiltrant phase has a controllable volume change upon solidification.
It is an object of the present invention to produce a composite body of increased thermal conductivity.
It is an object of the present invention to produce a composite body whose physical properties are at least somewhat tailorable by the presence of the additional metallic constituent(s) in the infiltrant material.
It is an object of the present invention to be able to produce a composite body at a temperature that is less than the melting point of pure silicon.
It is an object of the present invention to produce a composite body without having to rely on boron-containing materials or expensive molds being in direct contact with the preform or infiltrant material to control the extent of infiltration, e.g., “infiltration blockers”.
It is an object of the present invention to be able to produce composite bodies that are large, unitary structures.
It is an object of the present invention to be able to produce composite bodies of complex shape that are highly loaded in reinforcement material.
It is an object of the present invention to be able to produce composite bodies containing little to no in-situ silicon carbide phase.
It is an object of the present invention to be able to produce composite bodies in large numbers at a high rate of speed.
It is an object of the present invention to produce a composite body to near-net shape, thereby minimizing the amount of grinding and/or machining necessary to achieve the required dimensions of the finished article.
It is an object of the present invention to produce a composite body where any required grinding or machining can be performed substantially entirely at the preform stage.
It is an object of the present invention to produce a composite body where fine detail can be ground and/or machined into the body at the preform stage.